Mendel's Laws and Cross Patterns

Mendel’s Laws and Cross Patterns

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Topic 9 of the UNDANA Mandiri Saintek paper unifies Genetics and Evolution around Mendelian inheritance, the molecular basis of heredity (DNA, gene, chromosome), and population-level change through Hardy-Weinberg and natural selection. The must-know formulas are the two Hardy-Weinberg identities: p + q = 1 and p² + 2pq + q² = 1, where p = frequency of the dominant allele and q = frequency of the recessive allele, with p², 2pq, q² giving the frequencies of homozygous dominant, heterozygous, and homozygous recessive genotypes, respectively. A monohybrid cross of two heterozygotes (Aa × Aa) yields a phenotypic ratio of 3 : 1 and a genotypic ratio of 1 : 2 : 1. High-yield pointers: (1) test cross (× aa) reveals unknown genotypes; (2) X-linked traits (e.g. hemophilia, red-green colour blindness) appear predominantly in males because they are hemizygous; (3) evolution is defined as a change in allele frequency, not progress toward complexity.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Mendel’s Laws and Cross Patterns

Mendel’s First Law (Segregation): during gamete formation the two alleles of a gene separate, so each gamete carries only one allele. Mendel’s Second Law (Independent Assortment): genes located on different chromosomes are distributed into gametes independently of one another. A monohybrid cross of heterozygotes (Aa × Aa) gives genotypic ratio 1 AA : 2 Aa : 1 aa and phenotypic ratio 3 : 1 when A is dominant. A dihybrid cross of double heterozygotes (AaBb × AaBb) yields the classic 9 : 3 : 3 : 1 phenotypic distribution, but only when the two genes assort independently. A test cross mates the individual of unknown genotype with a homozygous recessive (aa); the offspring ratios (1:1 vs. all dominant) directly expose the unknown parent’s genotype.

Molecular Genetics: DNA → Protein

DNA is a double helix of antiparallel polynucleotides whose sequence of nitrogenous bases (A–T, G–C) encodes genetic information. A gene is a DNA segment that codes for a functional product, packaged with proteins into chromosomes. Gene expression proceeds in two steps: transcription (DNA → mRNA in the nucleus) followed by translation (mRNA → polypeptide at the ribosome), with each triplet codon specifying an amino acid. Mutations (point, frameshift, chromosomal) generate the raw genetic variation on which evolution acts.

Hardy-Weinberg Equilibrium

For a population not evolving, allele and genotype frequencies remain constant across generations. The model requires: no mutation, no natural selection, no migration, an infinitely large population, and random mating. Given p + q = 1 and p² + 2pq + q² = 1, knowing one frequency (e.g. q² from the frequency of recessive phenotypes) lets you compute all others. Deviations from expected frequencies signal that evolution is occurring.

Key Vocabulary

Genotype = the allele combination an organism carries; phenotype = the observable trait. Homozygous individuals carry two identical alleles, heterozygous individuals carry two different alleles. Dominant alleles mask recessive ones in heterozygotes.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Sex-Linked Inheritance and Pedigree Logic

Genes on the X chromosome show a distinctive criss-cross inheritance: an affected father passes the allele to all daughters (who become carriers) but to no sons. Males are hemizygous — a single recessive allele on their single X is expressed. UNDANA questions often use pedigree diagrams and ask you to deduce the inheritance mode (autosomal dominant, autosomal recessive, or X-linked recessive). The standard checklist: does the trait skip generations? Does it appear more often in males? Is a carrier mother the bridge between affected males?

When Hardy-Weinberg Fails

Real populations violate the model through: (1) mutation introducing new alleles, (2) natural selection raising or lowering fitness of genotypes, (3) genetic drift in small populations, (4) gene flow via migration, and (5) non-random mating (e.g. assortative mating or inbreeding, which inflates homozygotes at the expense of heterozygotes). Any detected deviation in observed vs. expected genotype frequencies can be attributed to one of these forces.

Evolution: Definition and Evidence

Evolution is a change in allele frequency within a population over time — individuals do not evolve, populations do. Darwin’s mechanism requires variation, heritability, and differential reproductive success. Lines of evidence commonly tested at UNDANA: the fossil record (transitional forms such as Archaeopteryx), homologous structures (shared ancestry, e.g. pentadactyl limb), analogous structures (convergent evolution, e.g. wings of bats vs. insects), biogeography (endemism on Galápagos finches), and biochemical homology (conserved cytochrome c across taxa).

Common Mistakes

Confusing genotype with phenotype, applying the 9:3:3:1 ratio to genes that are linked rather than independently assorting, forgetting that X-linked recessive disorders pass from carrier mothers to sons, and treating Hardy-Weinberg as a description of all populations rather than a null model used to detect whether evolution is happening.

Practice Prompts

In a population, 16% of individuals display a recessive phenotype. Using p + q = 1 and p² + 2pq + q² = 1, compute the frequency of heterozygous carriers.
A red-green colour-blind man marries a carrier woman. Construct a Punnett square and state the expected phenotypic ratio among their sons.

Content adapted based on your selected roadmap duration. Switch tiers using the selector above.